1. Field of the Invention
The present invention relates to a multi-color toner set comprising a plurality of color toners for developing latent electrostatic images to form multi-color images, with the attainment of substantially the same hot offset initiation temperature, durability, and image transfer performance thereof with respect to each color toner in the multi-color toner set. These color toners for the multi-color toner set can be produced by a conventional fusing and kneading method and a conventional pulverizing method.
The present invention also relates to a method of forming multi-color images, using the multi-color toner set.
2. Discussion of Background
In electrophotography, a latent electrostatic image is formed on a photoconductor comprising a photoconductive material, using various means, and the formed electrostatic image is then developed with a toner to a visible toner image, and when necessary, the developed visible toner image is then transferred to a sheet of paper, and fixed thereto with the application of heat and/or pressure thereto, or using a vapor of a solvent, whereby a hard copy can be obtained.
As methods for developing the electrostatic image to the visible image, using the toner, there are known, for instance, magnetic brush development method, cascade development method, and powder cloud development method. In any of these development methods, the step of fixing the toner image to a transfer sheet is a very important step. In particular, when the toner image is thermally fixed, using heat rollers, the toner image in a fused state comes into contact with the surface of the heat rollers, so that a phenomenon that the toner image adheres to the surface of the heat rollers and is transferred thereto, that is, a so-called offset phenomenon, may take place. However, the occurrence of such offset phenomenon should be prevented.
Conventionally, in order to prevent the occurrence of the offset phenomenon, the surface of an image fixing roller is made of a material having excellent releasability with respect to the toner, such as silicone rubber or fluorine-based resin.
Furthermore, conventionally, in order to prevent the occurrence of the offset phenomenon more effectively, and also to prevent the fatigue of the surface of the image fixing roller, the surface of the image fixing roller is covered or coated with a thin layer of a liquid which has high releasability with respect to the toner, such as silicone oil or a fluorine-based oil, which may be referred to as the offset preventing liquid.
The above-mentioned method is extremely effective for preventing the occurrence of the offset phenomenon, but has a problem that an apparatus for supplying the offset preventing liquid to the image fixing roller is required, which makes the image fixing apparatus complicated in mechanism. Furthermore, the step of coating the surface of the image fixing roller with the oil has the risk of bringing about the problems that the surface layers of the image fixing roller are separated and accordingly the life of the image fixing roller is significantly shortened while in use.
Instead of using such an oil supplying apparatus, there has been proposed a method of supplying an offset preventing liquid from toner particles themselves in the course of the image fixing step with the application of heat and/or pressure. More specifically, a releasing agent such as low-molecular-weight polyethylene, or low-molecular-weight polypropylene, is added to the toner particles and is caused to ooze from the toner particles at the image fixing step while heat and/or pressure is applied thereto.
By the above-mentioned method, the occurrence of the offset phenomenon can be in fact prevented at the image fixing step when heat and/or pressure is applied. However, it has been found that the releasing agent is released from the toner particles and adheres to the surface of carrier particles or a charging member, so that a so-called spent phenomenon takes place by which the charging performance of the carrier particles and the charging member is significantly lowered, as a result, the durability of the toner or the developer is markedly lowered.
It is also known that the presence of the above-mentioned releasing agent within the toner impairs the fluidity of the toner or the transfer performance of the toner to a transfer sheet.
In order to prevent the toner particles from becoming a film on a development sleeve and/or on a photoconductor, and also to prevent the occurrence of the spent phenomenon on the carrier and the deposition of the toner on a blade, the following proposals have been made so as to control the state of the toner itself and also the state of the releasing agent in the toner: For example, in Japanese Laid-Open Patent Application 3-243956, there is proposed the use of a toner having a 200 to 5000 .ANG. average lattice length of a primary peak of a small angle X-ray scattering (SAXS); in Japanese Laid-Open Patent Application 3-296067, there is proposed a toner at a cross section of which there are formed island-sea structures composed of a binder polymer and polypropylene, with a maximum diameter in a major axis of island portions formed by the polypropylene being in a range of 200 to 3000 .ANG., and an average interval among the islands being 1 .mu.m or less; and in Japanese Laid-Open Patent Application 5-45925, there is proposed a toner comprising toner particles on the surface of which there is deposited a releasing agent with a particle size of 100 to 500 .ANG., having a fusing initiation temperature--fusing termination temperature difference of 50.degree. C. or less, and a melting point of 60 to 180.degree. C.
In order to maintain the development performance and abrasion resistance of a photoconductor, there is proposed in Japanese Laid-Open Patent Application 5-197199 a toner on the surface of which there are dispersed polyolefin particles with a particle size of 0.01 to 0.5 .mu.m in an amount of 2 to 20%; in Japanese Laid-Open Patent Application 7-301951 a toner comprising a binder resin and a releasing agent with an SP value difference thereof being 1.5 or less; and in Japanese Laid-Open Patent Application 7-271095 a toner comprising a releasing agent with a crystallization degree in a range of 40 to 60% when the releasing agent is in the toner.
The above proposed toners have excellent spent phenomenon prevention effect and improved durability when used alone. However, these toners have a significant shortcoming that when used as a set of a plurality of color toners, properties such as hot offset initiation temperature, durability, and image transfer performance thereof differ in each color toner.